Generally, a steel sheet or an aluminum alloy sheet for a vehicle body of an automobile is formed into a predetermined shape by press forming or the like and thereafter undergoes an assembly operation and is then subjected to chemical conversion treatment. Incidentally, the chemical conversion treatment refers to treatment for depositing zinc phosphate on the surface of the steel sheet or the aluminum alloy sheet before coating. The chemical conversion treatment also serves as pretreatment for the coating. It is therefore necessary to uniformly deposit a sufficient amount of zinc phosphate on the surface of the aluminum alloy sheet in order to improve material corrosion resistance or coating film sharpness after the coating.
Then, in a case where zinc phosphate is not uniformly deposited on the surface of the aluminum alloy sheet when the aluminum alloy sheet is subjected to zinc phosphate treatment by the chemical conversion treatment, there is the possibility of a reduction in the corrosion resistance such as filiform corrosion resistance or coating film blistering resistance and a reduction in the coating film sharpness.
Incidentally, the chemical conversion treatment involves providing a series of surface treatments such as degreasing, rinsing, surface conditioning, zinc phosphate treatment, and rinsing, and such a series of process steps is called chemical conversion treatment operation. Then, under the zinc phosphate treatment, the surface of the aluminum alloy sheet undergoes an anodic reaction and a cathodic reaction which occur simultaneously in a zinc phosphate solution, as given below.Anodic reaction (1): Al→Al3++3e− (oxidation reaction, electron donating reaction)  (1)Cathodic reaction: 2H++2e−→H2 (reduction reaction, or electron accepting reaction)  (2)
When the cathodic reaction as above proceeds, hydrogen ions in the zinc phosphate solution can naturally be consumed to thus increase the pH of the solution; however, the water dissociation constant, [H+][OH−]=10−14, remains substantially constant. In other words, as soon as a reaction as given below proceeds through what is called the law of chemical equilibrium thereby to feed protons (H+) into the zinc phosphate solution, zinc phosphate is deposited on the surface of the aluminum alloy sheet.3Zn(H2PO4)2→Zn3(PO4)2↓+4H3PO4  (3)
Here, uniform deposition of zinc phosphate on the surface of the aluminum alloy sheet requires that the anodic reaction (1) and the cathodic reaction (2) occur uniformly. Examples of a typical method for uniformly depositing zinc phosphate by accelerating simultaneously the anodic reaction (1) and the cathodic reaction (2) include adjustment of the composition of alloy. Then, it is known that 5000 series aluminum alloys, in particular, are affected greatly by the influence of the amount of copper and hence a lower content of copper in the alloys leads to lower zinc phosphate treatability. (Refer to Patent Literature 1, for example.) Also, Patent Literature 1 states that an Al—Mg based alloy having a copper content of 0.10% or less is prone to be insufficient in the amount of zinc phosphate coating film formed.
Also, the methods for uniformly depositing zinc phosphate include an approach of enhancing pretreatment (e.g. degreasing, rinsing, and surface conditioning) for zinc phosphate treatment. Specifically, it is known that phosphate treatability is improved by treating an aluminum material made of an Al—Mg—Si based alloy with an acid containing fluorine ions. (Refer to Patent Literature 2, for example.) This aluminum material is suitable for use as a body material for automobiles and is excellent in corrosion resistance.
Further, the methods for uniformly depositing zinc phosphate include an approach of reexamining a manufacturing process for an aluminum alloy sheet thereby to improve beforehand a surface condition for purposes of chemical conversion treatment. Specifically, in a manufacturing method of Patent Literature 3, a sheet material made of an Al—Mg based alloy, an Al—Mg—Si based alloy or an Al—Cu—Mg based alloy is, first, continuously heat treated through a heating zone and a cooling zone. Subsequently to the heat treatment, the sheet material is then subjected to a process for removing a surface oxide film by an alkaline solution or an acid solution, and, thereafter, is continuously heated to a temperature of 40° C. to 120° C. and is immediately wound up in a coil. Thereby, an aluminum alloy sheet excellent in formability and zinc phosphate treatability and also excellent in baking finish hardenability for an alloy group having baking finish hardenability is obtained. Also, it is stated that the aluminum alloy sheet is suitably available for use as a structural member for transport equipment, such as an outer panel of an automobile, in particular.
Also, a method for uniformly depositing zinc phosphate by improving the alloy composition of a material in itself by adding an element for accelerating the above anodic reaction (1) or cathodic reaction (2) to an original slab of an aluminum alloy sheet is possible. Patent Literature 4 discloses an aluminum alloy containing, by weight percentage, 2 to 6% magnesium and 0.3 to 2.0% zinc, and having, as impurities, copper, the content of which is limited to less than 0.03%, iron, the content of which is limited to less than 0.4%, and silicon, the content of which is limited to less than 0.4%, and having the remainder being aluminum and unavoidable impurities, and having manganese (Mn), chromium (Cr), zirconium (Zr), vanadium (V), titanium (Ti) and boron (B) added as selective ingredients. There is a disclosure indicating that the aluminum alloy is excellent in formability and zinc phosphate treatability for a vehicle body panel of an automobile.
Further, Patent Literature 5 discloses an Al—Mg—Si alloy sheet containing not less than 0.05% but less than 0.3% by mass of zinc, in which the content of copper is limited to less than 0.05% by mass. Also, the surface of the Al—Mg—Si alloy sheet is provided with a zincate film of 0.1 to 1.5 g/m2. The zincate treated Al—Mg—Si alloy sheet has the zincate film formed by single treatment and excellent in adhesive properties. Also, it is stated that excellent zinc phosphate treatability and corrosion resistance can be imparted to the alloy sheet and the alloy sheet is suitably available for use as an outer sheet of an automobile, in particular.
Until now, the inventor of the present invention has made various discussions on 5000 series aluminum alloy sheets for automobile manufactured from a slab continuously cast by a thin slab continuous casting machine. Then, Patent Literature 6 discloses an Al—Mg alloy sheet excellent in continuous resistance spot weldability. The Al—Mg alloy sheet contains, by weight, 2 to 6% magnesium, 0.15 to 1.0% iron, and 0.03 to 2.0% manganese. Also, the alloy sheet is characterized in that a surface layer of the alloy sheet on the side thereof to be pressed against an electrode in resistance spot welding contains 4000 or more intermetallic compounds having a grain size of 0.5 μm or more, per square millimeter.
Also, the inventor of the present invention has disclosed an Al—Mg alloy sheet having excellent bake softening resistance in Patent Literature 7. The Al—Mg alloy sheet contains 2 to 5% by mass of magnesium, more than 0.05% but not more than 1.5% by mass of iron, and 0.05 to 1.5% by mass of manganese, and the total content of iron (Fe) and manganese (Mn) exceeds 0.3% by mass. Also, the alloy sheet is characterized in that the content of iron in a solid solution is equal to or more than 50 ppm, in that intermetallic compounds with 1 to 6 μm of an equivalent circle diameter are present for 5000 or more per square millimeter, and further, in that an average sizes of recrystallized grains is 20 μm or less.
Further, the inventor of the present invention has provided an Al—Mg alloy sheet having excellent deep drawability and bake softening resistance as disclosed in Patent Literature 8. The Al—Mg alloy sheet contains, by mass, 1 to 5% magnesium, 0.1 to 1.0% iron, 0.005 to 0.1% titanium, 0.0005 to 0.01% boron, and unavoidable impurities, and the content of silicon in the unavoidable impurities is set to be less than 0.20%. Also, the alloy sheet is characterized in that the solid solution content of iron in a matrix is 50 ppm or more, in that the diameter of recrystallized grain is 12 μm or less, and in that a limited drawing ratio is 2.13 or more.
However, none of Patent Literatures 6 to 8 have made mention of chemical conversion treatability.